A team led by researchers from Sandia National Laboratories and the University of California, Merced has developed an efficient molybdenum disulfide (MoS2) catalyst for driving the hydrogen evolution reaction (HER). In a study published in the journal Advanced Materials, the team reports that metastable and temperature-sensitive chemically exfoliated MoS2 (ce-MoS2) can be made electrochemically stable (5000 cycles), and thermally robust (300 °C) while maintaining synthetic scalability and excellent catalytic activity through physical-transformation into 3D structurally deformed nanostructures.

The deformed nanostructures, produced using a novel spray-printing process, resemble a plant’s leaves; the increased surface area of the rippling “leaf” creates three times as many catalytic contact points as other molybdenum disulfide structures, and the new creation can handle higher temperatures than platinum without sintering and gumming up the cell.

These inorganic “flowers,” color added, were created by Sandia National Laboratories researcher Stanley Chou and University of California, Merced colleague Vincent Tung in a spray-printing process that uses molybdenum disulfide to create a “flowering” hydrogen catalyst far cheaper than platinum and reasonably close in efficiency. (Image courtesy of Sandia National Laboratories) Click to enlarge.

While Pt nanoparticles represent the current benchmark for HER electrocatalysis, the elemental scarcity, high cost, and poor stability against nanoparticle coarsening and agglomeration impose practical limitations and motivate the need for a next generation of alternative HER catalysts. MoS2, a van der Waals solid composed of adhered-2D monolayers, has emerged as a promising alternative to Pt due to its elemental abundance, high catalytic activity, and electrochemical stability. Despite the various merits of MoS2, its functional performance remains inferior to that of Pt due to comparatively small active site concentrations as well as its relatively low electrical conductivity.

At the root of functional limitations is the inability of MoS2, in its natural 2H semiconducting phase, to catalyze HER on its inert basal plane, where the Gibbs free energy for hydrogen adsorption (∆GH) is exceedingly large. In contrast, MoS2 edges possess a favorable ∆GH, as well as metallic 1D conductivity due to the presence of electronic states in close proximity to the Fermi level. Because of the highly catalytic nature of MoS2 edges, synthetic approaches that prioritize MoS2 edges sites over the inert basal plane sites have attracted broad interest. … Yet, despite these innovations, the fundamental inactivity of the MoS2 basal plane remains a basic material constraint.

Lead researchers Stanley Chou, a Sandia materials scientist, and UC Merced’s Vincent Tung have applied for a joint patent for the spray-printing process. The production method uses nature as an ally rather than a hindrance, Chou said. Tung said the method uses natural processes to produce materials for extremely inexpensive fuel cell terminals to liberate hydrogen. The printing process also allows for continued deposition, with the ability to scale for industry, he added.

The team mixed molybdenum disulfide with water and used the printing process to expel micron-size droplets into an enclosed area about 2 feet high. As they dropped, the droplets first separated into nanoscopic subunits. These dried further as they fell, their shrinking volume producing an uneven 3-D surface much like the leaves of plants, with tiny ridges, hills, canals, caves and tunnels.

Landing on a substrate and on each other, the “leaves” were still moist enough to bond as though attached at critical points by tiny droplets of glue. Thus, the nanostructures did not lose their individuality but instead, by maintaining their identities, created tiny tunnels within and between them that permitted extraordinary access for atoms of hydrogen to seek their freedom from chemical bonds.

The use of electrostatically charged droplets as nanoreactors facilitates colloidal dispersion, fission, and capillarity-induced-self-crumpling of 2D ce-MoS2, ultimately transforming it into 3D c-MoS2.

—Chen et al.

The inspiration for creating a bio-inspired 3-D form arose from studying the cuticle folding process, a mechanism used by plants for controlling diffusion and permeability on leaf surfaces, Chou said.

We see our catalyst as an inorganic material acting like a plant. The nanostructures, like leaves, are varied in shape, with tiny rises and falls. The structures take in an external material to produce hydrogen rather than oxygen, and one day may be powered by sunlight.

—Stanley Chou

Right now, very low-voltage electricity does the job.

Doubts about the strength of structure formed in such a serendipitous manner, Tung recounted, were settled when a 170-pound student unwittingly trod upon one of the first molybdenum disulfide-catalyst creations when it accidentally fell to the floor. A few hundred nanometers thick, it rested upon a centimeter-square carbon substrate but was otherwise unprotected. Elecromicroscopic investigation showed the tiny structure to be undamaged. The “leaves” also have proved to be long lasting, continuing to produce hydrogen for six months.

…our work provides new insights into the enhancement of catalytic activity, electrochemical and thermal stability of MoS2 catalysts through physical transformation that is reminiscent of nature, in which properties of biological materials emerge from evolved dimensional transitions. Given the wide variety and availability of chemically exfoliated 2D functional materials, including graphene, transition metal dichalcogenide (TMD), and the emerging MXenes, we anticipate that structurally robust, thermally stable, electronically heterogeneous, and catalytically active, multifunctional hybrid nanocomposites that were previously unattainable can now be readily, rapidly, and rationally assembled through this template free, and scalable nanomanufacturing route. This, in turn, provides exciting opportunities for numerous applications beyond HER, including electrochemical capacitors, battery, catalysis, reactors and separation, drug delivery, biocompatible scaffolds, sensing
and high complexity composites.

—Chen et al.

Researchers from King Abdullah University of Science and Technology, Lawrence Berkeley National Laboratory and Yale University also contributed to the article.

The work at Sandia was funded by the Department of Energy’s Office of Science. Work at UC Merced was supported by a university startup fund.